The electrochemical transformation of CO2 into formic acid and vice versa is of great interest due to its unique features: HCOOH formation is the formally simplest case of CO2 recycling, requiring only two electrons and two protons while leaving the CO2 skeleton intact. Formic acid is largely used in agriculture, but it also has important chemical applications: HCOOH is used as a safe and convenient equivalent of H2 in transfer hydrogenations. Moreover, the rather non hazardous liquid has a high volumetric hydrogen density and could even be used in direct fuel cells. Here we investigate the kinetics of the formation and decomposition of formic acid under electrochemical conditions. Despite recent progress in modelling electrocatalytic reactions, there is still no consensus on how to assess the effect of the electrode potential at an atomic scale. In the popular and efficient approach of Norskov[1] activation energies do not depend on the electrochemical potential. By comparing with results that account explicitly for the electrochemical potential[2] and applying an implicit solvent model[3], we demonstrate that the potential dependence of reaction barriers should not be neglected. While some models assume that the potential dependence is related to the breaking of the X-H bond, we find that the largest potential dependence occurs for “innocent” looking reorientations of adsorbed formate. The potential dependence reveals a competition between the reorientation and the desorption as the highest energy transition states. The dipole moment of the adsorbate normal to the electrode surface is strongly correlated to the workfunction. Furthermore, the potential dependence is directly related to the difference in workfunction between the initial state and the transition state. In addition to guidelines on how to judge whether going beyond the Norskov approach is necessary when modelling electrochemistry, we emphasize the need to model an entire pathway, including desorption/adsorption steps that can have significant barriers under electrochemical conditions. [1] J. K. Norskov, J. Rossmeisl, A. Logadottir, L. Lindqvist, J. R. Kitchin, T. Bligaard, H. Jonsson, J. Phys. Chem. B 2004, 108, 17886. [2] M. Mamatkulov, J. S. Filhol, Phys. Chem. Chem. Phys. 2011, 13, 7675. [3] K. Mathew, R. Sundararaman, K. Letchworth-Weaver, T. A. Arias, R. G. Hennig, J. Chem. Phys. 2014, 140, 084106.

Modeling the HCOOH/CO2 electrochemical couple: When details are key

249th National Meeting (2015)

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